The Parameters Calculation And Optimum Design Of Grounding Grids Of Large Substation

The grounding system is one of important components of a substation, to which much attention is always paid. The substation grounding grid can provide a reference ground for various electric devices within the substation. During a fault of an electric power system, the failure current drains quickly to the ground. The maximum ground potential rise can be controlled to ensure adequate safety for people and to protect the installation. Therefore, an effective grounding grid is very important for the safe operation of the electric power system. Along with the development of electric power system capacity, the value of failure current flowing into ground has been increased greatly. Particularly, after the construction of the three gorge power plant, 34kA can be reached. In order to control ground potential rise, the area of the grid may be very large and maximum diagonal of the grid can be above a kilometer. Because the area of large substation is big, it is inequipotential. The gradient of earth surface potential above the large equidistant substation is big, and the leakage current of theirs conductors are not uniform. It is very essential that the grounding grids of large substation using variable spacing be researched.The main achievements are as follows:1. A fast approach to Green function in multi-layer earth and its partial differentials to the earth parameters is introduced in detail by using the complex image method. An efficient method for estimating earth structure from Wenner's four-probe test data is presented. Both of them provide theoretical foundation for analyzing performance of grounding grids in power current.2. Based on method of moment and field-circuit coupling theory, nodal method of analysis in circuit theory is applied to the power parameters calculation of grounding grids. The inner self-resistance and self-inductance of conductors, mutual inductance between conductors, and displacement current of conductors are taken into account. The methods are able to compute any parameter of grounding grids in power current, such as power ground impedance, touch and step voltage, potential differences within grounding grids and so on. Among current methods analyzing grounding grids in power current, the nodal method of analysis is the easiest and least exponent number of matrix in calculation.3. In order to reduce ground resistance of grounding grids, in project, measures are generally replacing steel material grounding grids by copper and subsidiary groundinggrids. The paper compares in detail the ground performance of large copper and steel material grounding grids in power current, which has some reference for the practical engineering. According to experience, the distance between subsidiary grid and the main grounding grids is 1-2 km, both grounding grids are generally connected with buried conductors. The paper gives some scientific foundation for practical engineering. 4. To equalize the potential and decrease touch voltage and step voltage on the earth surface, an optimum design of grounding grids is proposed in this paper. Based on equipotential models of some scholars, according to a practicable fact which large substation grounding grids are not equipotential any longer, in inequipotential model, the paper analyzes the influence of the soil resistivity and the grounding grid area on the optimum arrangement in the uniform soil; the paper analyzes the reflective coefficient of double-layer soil, the top-layer soil thickness and the grounding grid area on the optimum arrangement. The paper establishes the object function by the minimum difference between the maximum and the minimum touch voltage, in orderto obtain the optimum compression ratio, the paper uses the numerical method------thevariable metric method to analyze the obtained many data. The paper obtains the formula to calculate the optimum compression ratio of the optimum arrangement in the uniform and double-layer soil, which presents some reference to design the large substation grounding grids.